The present invention relates to a method of recovering chemicals from chloride-containing green liquor. This invention relates in particular to a method of recovering the digesting and bleaching chemicals used in paper making. The method according to the present invention is especially usable in pulp mills in which a closed cycle of the bleaching chemicals is used. In such closed systems, chloride tends to concentrate and corrode the apparatus, unless it is removed in some way.
The object of the present invention is thus to provide a method for recovering chemicals, and especially hydrogen sulfide, sodium carbonate and sodium chloride, from a chloride-containing green liquor which has been obtained by burning a mixture of black liquor and chloride-containing solutions obtained from the bleaching.
The removal of sodium chloride from closed sulfate cellulose processes is described in, for example, U.S. Pat. No. 3,698,995, No. 3,746,612 and No. 3,909,344. The efficiency of these methods is, however, limited; they require a large amount of steam for evaporation and are expensive in investment.
U.S. Pat. No. 4,138,312 describes a method for the recovery of sodium carbonate from the chloride-containing waste liquor from soda cooking, the sodium carbonate being crystallized in the form of a monohydrate and being carbonated thereafter.
The object of the present invention is thus to provide a method, more efficient than previously, for the recovery of chemicals from chloride-containing green liquor, and in particular from a green liquor which has been obtained by burning black liquor derived from sulfate digestion and a chloride-containing bleaching solution. In addition, the other sodium chemicals can be recovered in a substantially chloride-free form so that, after causticization, they can be used for preparing white liquor. By the method according to the invention it is possible to separate the chlorides in crystalline form, as sodium chloride, from which it is easy to prepare a new bleaching solution. In addition, by the method according to the invention the hydrogen sulfide formed from the sodium hydrosulfide present in the green liquor can be recovered with maximum efficiency and be converted to sulfide chemicals suitable for the production of white liquor.